Since 2005, the installed wind power capacity has kept increasing rapidly, and annual increase of the installed wind power generating capacity is more than 20% during 2005-2011. Since 2011, development of wind power has entered into a stable period, and annual increased wind power generating capacity is approximately 40 GW. However, a bottleneck of rapid wind power development is that most wind power bases are located in remote areas where power load of local AC power grids is low, self-consumption thereof is poor, and wind energy is adversely distributed with respect to power demand, which make it necessary to transfer most wind power via large-scale wind power transmission for consumption at a load center. High voltage DC transmission (HVDC) is a commonly-used technology for the large-scale power transmission over long distance.
Prior to transmission by the HVDC, wind power needs to be firstly converted from AC power to DC power by a converter. In the field of the HVDC, devices for AC-DC conversion mainly comprise line commutated converters (LCC) based on thyristors, and voltage source converters (VSC) based on fully-controllable power semiconductors. Since in operation, the LCC requires an external AC voltage source to provide commutating voltage thereto, while a wind farm is a passive AC system, and it is difficult to establish AC voltage using multiple wind power generators that are individually distributed, therefore, the LCC is not suitable to directly convert the wind power to DC.
The voltage source converter can be applied to wind power transmission, and a number of HVDC systems have been used to transmit offshore wind power. However, there has been no report of applying the voltage source converter for transmission of inland wind power over long distance.
Applying the voltage source converter for transmission of inland wind power over long distance faces a main challenge that rated voltage and rated power of the voltage source converter cannot meet the requirement for long distance transmission of inland wind power. It is respected that by the year of 2015, typical voltage and power ratings of the voltage source converter will be ±320 kV and 1000 MW, respectively. At present, the voltage source converter mainly operates to transmit off-shore wind power with a transmission distance of approximately 100 km.
In long-distance power transmission over thousands of kilometers, if the rated DC voltage of a HVDC system is low, power that can be transmitted by each HVDC transmission line will be low; which makes it difficult to recover investment in constructing a transmission corridor by profit obtained from the HVDC system. Meanwhile, as DC voltage reduces, the DC current required to transmit the same amount of DC power will become higher, and power loss will be increased. Therefore, in long distance transmission over thousands of kilometers, a higher rated DC voltage will be required to improve transmission power and reduce the power loss during transmission. The LCC is used in long distance HVDC transmission over thousands of kilometers, and typically rated voltage thereof is ±800 kV. It can be seen that the rated voltage of the voltage source converter cannot meet requirement for large scale power transmission over long distance.
With the development of technology, the rated voltage of the voltage source converter in the future may be up to ±800 kV. Since the voltage source converter adopts the full-controllable power semiconductor with rated power of each semiconductor significantly lower than a single thyristor, to enable the rated voltage of the voltage source converter to meet the requirement for long distance transmission, a multiple full-controllable power semiconductors need to be connected in series (in direct serial connection or indirect serial connection) to increase the rated voltage of the voltage source converter. If the number of the full-controllable power semiconductors becomes large, complexity of the system will be increased, system reliability will be reduced, and the rated power of the voltage source converter still cannot meet the requirement for long distance transmission. To enable the rated power of the voltage source converter to meet the requirement for long distance transmission, a non-conventional scheme is to employ multiple lower rated voltage and high rated current full-controllable power semiconductors connected in series to increase the rated voltage and the rated power of the voltage source converter. If such scheme is used, the number of fully-controllable power semiconductors that are used will be significantly increased, which further increases system complexity and reduces system reliability.